Process of producing sintered tungsten electrodes



L. PERA'S Oct. 13, 1959 PROCESS OF PRODUCING SINTERED TUNGSTEN ELECTRODES Filed Jan. 31, l 1955 FIG 2 FIG 4' FIG 3 surface and the central electrode being held at its United States Patent PROCESS OF PRODUCING SINTERED TUNGSTEN ELECTRODES Lucien Peras, Billancourt, France, assignor to Regie Nationale des Usines Renault, Billancourt, France Application January 31, 1955, Serial No. 485,180 Claim priority, application France February 5, 51954 1 Claim. (Cl. 75-207) This invention relates to improvements in spark plugs, more particularly to spark plugs for mternal combustion engines.

The invention relates to a new constructional form of spark plugs which are very suitable for ignition by capacity discharge and whose working life is much greater ice and to braze each of the electrodes end-on to a conducting member of, for example, steel.

Several constructional forms of spark plugs particularly suitable for electrodes according to the invention are shown in partial axial section in the accompanying drawings purely by way of example.

Figs. 1 through 5 are fragmentary sectional views of spark plugs according to the invention,

Fig. 6 is an elevation 'view of a spark plug element partly insection.

Figs. 1 and 5 show a spark plug comprising electrodes I 1,2 with insulating material 3 which is for example cethan that of spark plugs which have been constructed heretofore, and of the same order as the life of the currently used high voltage spark plugs.

The spark plugs according to the present invention are constituted by co-axial circular electrodes machined from special alloys, the dimension of the electrodes being much greater than that of the region subjected to the sparks. The metals from which the electrodes are machined are alloys rich in tungsten obtained by impregnation of calcined tungsten bodies of carefully adjusted porosity by another metal or alloy having particular properties which is brought to a molten condition for the impregnation process. The tungsten content of the alloys which are suitable is comprised between 65% and 95%. According to the invention, the complementary metal added by impregnation in the molten condition can be:

pure silver, pure palladium,

- palladium-silver alloys in any proportions.

A moderate melting temperature of less than 1,400 degrees centigrade permitting of the easy impregnation of the tungsten body is reconciled with good properties of resistance to the spark by using alloys containing 70% palladium and 30% silver, or else 50% palladium and 50% silver.

For example, in tests carried out on a vehicle having a gasoline engine of current construction, wherein the ignition of the four 748 cm. cylinder engine is effected by low voltage (800 to 1,000 volts approx) The lifedistance of the spark plugs as hitherto used was scarcely more than 5,000 kilometres.

With a spark element wherein the electrodes, spaced from one another by 0.12 mm. by mica whose mechanical action is reinforced by an insulating cement, are formed of an alloy consisting of 85% tungsten and 15% silver, it was possible to travel 18,000 kilometres with the same vehicle under the same running conditions as those in the preceding test before it was necessary to change the spark plugs. The distance travelled could have been increased to 25,000 kilometres with another set of spark plugs wherein the electrodes consisted of an alloy of 80% tungsten, 9.8% palladium and 4.2% silver. In both cases the spark element was held in a mounting which enabled ordinary high voltage spark plugs to be fitted at the appropriate place and time, the peripheral electrode being connected to the frame by its entire outer center by a spindle which is in turn connected to the other pole of the current source.

With a view to reducing costs, it is possible to reduce the height of the spark element to, for example, 3 mm.

ment vitrified into position or cement, consisting of a refractory mineral charge agglomerated by a heat-hardened organic substance, and a mica ring 4 arranged at the height of the region where the sparks are produced. The'mica ring protects the insulating material and supports the fire o f the spark.

In other constructional forms (see Figs. 2, 3 and 4), a circular swelling or curvatureS can be formed at the top of one of the cylindrical electrodes in such manner that the shortest distance between electrodes is outside the insulating material and that the spark, jumping beyond the said insulating material, does not damage it.

In another possible constructional form based on prolonged'tests, electrodes of pure tungsten were employed 7 provided that the volume of pure tungsten was substantiaily greater than that which is subjected to the spark.

For example, it would not be advantageous to cause the spark to jump between two tungsten toruses each constituted by a wire placed in the form of a circle and secured by brazing to a cylindrical steel part. The electrodes formed in this known manner can provide, for example in motor cars, only a quite inadequate service life, even if the diameter of the tungsten wire is at the upper limit of approx. 0.25 mm. permissible for winding and brazing to a steel 'n'ng w'hose thickness is of the order of 0.6 mm.

In order to define precisely the features of the invention, reference will now be made to the example of a spark plug element (Fig. 6) for a gasoline engine. The electrodes 1 and 2 are connected respectively to the electric generator (not shown) by the crown 6 and the cylindrical stem 7. The two electrodes are insulated from one another and held in position by the cement 3. The spark controlled by the distributor jumps at any point in the inter-electrode space between two points such as A and B situated on the same diameter.

Whereas the small toroidal surfaces which have to be subjected to the spark, taken in section at A and B through the plane of the drawing, have a width not exceeding 0.1 mm., the tungsten electrodes used by way of example and in accordance with the invention have a thickness of 1 mm. and a height of 5 mm. According to the invention, no material discontinuity of the brazed connection, for example, must exist within each of the electrodes at less than 2 mm. from the line of sparking.

In order that the service life of the electrodes s'hould fulfill the requirements of, for example, automotive use, the tungsten must not be porous and its compactness should be such that its specific weight should be 'greater than 17.5 g./cm. and preferably greater than 18 g./cm. This condition is diflicult to fulfill since the only tungsten products commercially obtainable which exhibit no porosity are those wires whose manufacture has included the preparation of a rod by calcining, then a hammering and a series of wire-drawings, which are mechanical treatments to which spark plug electrodes cannot be subjected during manufacture in view of their form and their size.

The invention also relates to a method of preparing pure tungsten electrodes for spark plugs operating with capacity discharge, having the compactness necessary for good operation in practice. It comprisesthe following conditions taken necessarily in combination:

Use of a pure tungsten powder of suflicient fineness for the specific weight of the rammed powder to be less than 3 g./cm.

Shaping by moulding under pressure, preferably by double compression in a floating matrix, the moulding pressure being greater than 7 kg./mm. although low enough to prevent the appearance of cracks in the moulded article. A suitable pressure is one between 7 and 10 kg./mm. I

Calcining at a temperature of between 1,800 and 2,100 C. by heating in a graphite resistance furnace in an atmosphere consisting of a mixture of argon and hydrogen, the hydrogen content being less than 10%. In another embodiment it is possible to place the compressed article to be sintered in a cold furnace, heat to 1,100" in'an atstrong a content has the result of carburizing the tungsten (probably through the intermediary of gaseous hydrocarmosphere of pure hydrogen, stop the current of hydrogen at this instant and substitute therefor a current of'pure argon. It is sufficient to maintain this temperature for 15 to 30 minutesv Sintering can be carried out by heating the powder electrode in an atmosphere of pure hydrogen up to a temperature between 1,000 centigrade and 1,300 centigrade and then in an atmosphere of argon up to a maximum temperature of 2,100 Centigrade.

The hydrogen is necessary for ensuring Igood connection between the grains of tungsten, probably because it effects the reduction of oxide traces which may cover the metallic grains. It has been found, moreover, that the use of hydrogen at too high a temperature and in too bons formed by the action of the hydrogen on the graphite of the furnace) and of making it fragile, unmachineable and of low resistance to sparking.

The electrodes are then machined to their final size.

I claim:

The process of making low voltage discharge device electrodes, which comprises, providing pure tungsten powder having a specific weight less than three grams per cubic centimeter, molding the powder into electrodes in a matrix under pressure between the limits of seven kilograms per square centimeter and ten kilograms per square centimetergheatinsg the electrodes in the presenceof pure hydrogen to a temperature between 1000 centigrade and 1300 centig'rade and then heating them to approximately 2100 centigrade in the presence of argon thereby to form non-porous electrodes.

References Cited in the file of this patent UNITED STATES PATENTS 2,008,617 ,Lampitt July 16, 1935 2,069,951 Hastings Feb. 9, 1937 2,253,533 Ruben Aug. 26, 1941 2,311,647 Doran Feb. 23, 1943 2,491,866 Kurtz Dec. 20, 1949 2,578,754 Smits Dec. 18, 1951 2,625,922 Smits Ian; 20, 1953 2,675,310 Hall Apr. 13, 1954 2,689,556 Smits Sept. 21, 1954 2,712,685 Johnson et a1. July 12, 1955 2,791,022 Stuermer May 7, 1957 

